1. Field of the Invention
The invention relates to a hydraulic control circuit for a boom cylinder in construction and/or civil engineering machinery, such as a hydraulic excavator that is used in various types of work machines.
2. Description of Related Art
Generally, in some construction and civil engineering machinery, such as a hydraulic excavator, there is a type in which a front attachment mounted on the machinery body is composed of a boom whose base end portion is supported so as to swing upward and downward, an arm that is supported at the tip end portion of the boom so as to swing forward and backward, and a work attachment, such as a bucket attached to the tip end portion of the arm. In such a piece of machinery, the above-described boom moves upward and downward in line with extension and contraction of a boom cylinder, wherein, conventionally, a hydraulic control circuit, such as shown in FIG. 3, has been known as one of the hydraulic control circuits for the boom cylinder.
That is, in FIG. 3, reference number 8 denotes a boom cylinder, reference numbers 9, 10 denote other hydraulic actuators in addition to an arm cylinder and a bucket cylinder, which are attached to a hydraulic excavator. Reference number 11 denotes a pressurized oil supply source for the boom cylinder 8 and other hydraulic actuators. Reference number 12 denotes an oil reservoir. Reference number 13 denotes a boom control valve for controlling the supply and discharge of pressurized oil with respect to the boom cylinder 8. Reference numbers 14, 15 denote other hydraulic actuator control valves for controlling the supply and discharge of pressurized oil with respect to the other hydraulic actuators. Reference number 16 denotes a pilot valve for outputting pilot pressure to pilot lines C, D at the elevation side and descent side on the basis of the operation of a boom operating lever 17. In addition, reference letter A denotes the head side line for connecting the boom control valve 13 to the head side oil chamber 8a of the boom cylinder 8. Reference letter B denotes the rod side line for connecting the boom control valve 13 to the rod side oil chamber 8b of the boom cylinder. Also, reference letter G denotes a recycling line for communicating the above-described head side line A to the rod side line B, and a recycling valve 27, provided with a check valve 27a, is provided in the recycling line G.
In this type of hydraulic control circuit, when the boom operating lever 17 is operated to the descent side, with the pilot pressure outputted from the pilot valve 16 to the descent side pilot line D, the boom control valve 13 is changed to the descent side position Y. At the same time, the recycling valve 27 is changed to the second position Y that opens the recycling line G, wherein, while pressurized oil from the pressurized oil supply source 11 is supplied to the rod side oil chamber 8b of the boom cylinder 8 via the boom control valve 13, oil discharged from the head side oil chamber 8a is discharged into the oil reservoir 12, and is further supplied to the rod side oil chamber 8b via a recycling valve 27. That is, while the pressure of the head side oil chamber 8a is higher than the pressure of the rod side oil chamber 8b when the boom is lowered, the oil discharged from the head side oil chamber 8a can be provided to the rod side oil chamber 8b as recycling oil, whereby the recycling oil is supplied into the rod side oil chamber 8b in addition to the pressurized oil of the pressurized oil supply source 11, which is provided from the above-described boom control valve 13. Therefore, the rod side oil chamber 8b is not placed into a pressure-reduced state, wherein the operation speed of the boom cylinder 8 can be accelerated. Also, when other hydraulic actuators (for example, arm cylinder and bucket cylinder) for which the pressurized oil supply source of the boom cylinder 8 is concurrently used are operated in combination with a descending operation of the boom, because surplus pump flow volume obtained by the recycling can be provided to the other hydraulic actuators, it is possible to prevent the operation speed of the other hydraulic actuators from being lowered when such a complex operation is carried out, wherein the surplus pump flow volume obtained by the recycling can contribute to an improvement in work efficiency.
However, because a force against the descent of the boom operates where the above-described boom is lowered for execution of rolling work by descent of the boom and scraping work of an inclined plane, it is necessary to supply highly pressurized oil into the rod side oil chamber. To the contrary, when the boom is lowered in the air (where the boom is lowered in a state where the front attachment is not grounded), because a weight applied to the boom (the total weight of the front attachment) operates as a force for contraction of the boom, the pressurized oil supplied into the rod side oil chamber may be low. In addition, as the head side area of a piston of the boom cylinder is larger than the rod side area thereof, only the recycling oil from the above-described head side oil chamber may be sufficient.
Accordingly, where the boom is lowered with the above-described related art hydraulic circuit, not only is recycling oil provided but also pressurized oil from the pressurized oil supply source is provided via the boom control valve even if the boom is lowered in the air. Therefore, there is a problem in that, in cases of operating the arm and bucket while the boom descends in the air, pressurized oil from the pressurized oil supply source is shared among the boom cylinder, arm cylinder and bucket cylinder, wherein the movement of the arm and bucket becomes slow in comparison with independent operations thereof, resulting in a worsening in work efficiency. Further, in spite of only the recycling oil from the head side oil chamber being sufficient where the boom is independently lowered in the air, because pressurized oil is supplied from the pressurized oil supply source to the rod side oil chamber, considerable surplus oil is discharged from the head side oil chamber 8a to the oil reservoir 12 via the boom control valve 13, wherein an energy loss is produced, which hinders a decrease in fuel consumption. These and other problems are objects to be solved by the invention.
Further, in a work machine provided with a front attachment consisting of the above-described boom, arm, work attachment, etc., for example, in a case where work for raking up debris is carried out while moving the bucket forward and backward with the bucket bottom grounded, it is necessary that three operations of the boom, arm and bucket so as to cause the bucket bottom to depict a roughly horizontal locus are simultaneously carried out. The operations are delicate, and skilled operations are required. In addition, where the ground is hardened with the bucket bottom by continuously repeated operations of descent and elevation of the boom, that is, bumping work is carried out, unless the boom elevation operation is timely carried out at the instant when the bucket bottom is grounded, the ground is excessively bumped by a reaction caused at the time of descent of the boom, or the front part of a machine body is raised. Therefore, in order to continuously carry out bumping, a highly skilled operation is required. The work requiring such skill is difficult for a beginner, and even a skilled operator is obliged to pay meticulous attention to the work. Accordingly, another problem to be solved exists in that, in such situations, the maneuverability and work efficiency are made worse.
In view of the above-described situations, the invention was developed to solve these and other problems. It is therefore an object of the invention to provide a hydraulic control circuit for a boom cylinder, which is provided with a boom cylinder extending and contracting so as to move a boom upward and downward and a control valve that is changed, on the basis of operation of an operating member, to an operation position at which pressurized oil is supplied to or discharged from respective oil chambers at the head side and the rod side of the boom cylinder, and to a neutral position at which no pressurized oil is supplied and discharged, and further comprises means for holding the control valve at a neutral position regardless of operation of the operating member, and an oil communicating line for communicating the head side oil chamber and the rod side oil chamber of the boom cylinder with each other, wherein the above-described oil communicating line includes valve means for opening and closing the communicating line, and a direction valve means that is changed to an unidirectional state where a reverse flow is hindered although an oil flow from an oil chamber at the weight-holding side to hold the weight of the boom of the respective oil chambers of the boom cylinder to the other oil chamber is permitted, and a bi-directional state is permitted where oil flows in both directions.
And, with such a structure provided, as in line with work contents carried out by the boom, the control valve can be held in a neutral state and the oil communicating line can be opened and closed in an unidirectional state or a bi-directional state, this contributes to a decrease in fuel consumption, and at the same time, work efficiency can be improved in combination work with other hydraulic actuators for which the pressurized oil supply source of the boom cylinder is concurrently used, or work efficiency and operationality can be improved with respect to work, such as debris raking-up work, or bumping work, for which the boom operations are difficult.
In such a structure, the hydraulic control circuit is provided with discharging means for flowing surplus oil of the oil discharged from the weight-holding side oil chamber of the boom cylinder and supplied into the other oil chamber to an oil reservoir when the control valve is held at the neutral position by the neutral holding means, wherein surplus oil of the oil discharged from the weight-holding side oil chamber flows into the oil reservoir even if the control valve is held at the neutral position, wherein such an inconvenience, by which the discharge of oil from the weight-holding side oil chamber is hindered and the descending speed of the boom is made slow, can be prevented from occurring.
Also, in such a structure, pressure detecting means for detecting pressure in the other oil chamber is provided, and where the pressure of the other oil chamber, which is detected by the corresponding pressure detecting means, is lower than a predetermined pressure, the neutral holding means operates so that the control valve is held in the neutral position, wherein, for example, when the boom is lowered in the air in line with a descending motion of the boom recognized by the pressure of the other oil chamber, the neutral holding means operates to cause the control valve to be held in the neutral position.
Further, the above-described neutral holding means may be composed of, for example, valve means that hinders pilot pressure outputted so that the control valve is operated to the operation position on the basis of operation of an operating member.
Still further, where pressure detecting means for detecting the pressure of the other oil chamber is provided, and the pressure of the other oil chamber, which is detected by the corresponding pressure detecting means, exceeds the predetermined pressure, the direction valve means is set so that a unidirectional state is not changed to a bi-directional state, wherein it is possible to prevent the communicating oil line from being carelessly placed into a bi-directional state, for example, in a case where the front part of the machinery is raised by the boom descending force.